Author John Shirley pointed us to his blog where he talks about the Constantine novelization:

I wrote the novelization of the Hellblazer movie and I talk about the script a bit and my novelization and about forthcoming HELLBLAZER NOVELS based on the comics at the centerblog, JohnShirley.net – I am the cowriter of the movie THE CROW as well as the author of the novel DEMONS and the Bram Stoker winning collection BLACK BUTTERFLIES.